Automatic transmission fluids are lubricants used in motor vehicle transmissions. Different types of automatic transmission fluids are used depending on the design and severity of application. Generally, automatic transmission fluids are designed to meet specific manufacturer requirements. An automatic transmission is composed of a complex variety of mechanical parts which operate at close tolerances. The purpose of automatic transmission fluid is to lubricate these close-fitting parts to reduce wear and keep down temperature due to friction. To perform this function, the automatic transmission fluid must maintain its viscosity within certain specifications. Achieving this function is complicated by the changing temperatures under which the transmission is operated. It is desirable that an automatic transmission fluid perform well in all the various temperature conditions under which the transmission will operate. For example, in a Northern location, the automatic transmission fluid may be below 32.degree. F. prior to use and then heat up to over 300.degree. F. during use. Mineral oil based lubricating oils typically have higher viscosities at lower temperatures and lower viscosities at higher temperatures. As automatic transmissions evolve, however, the maximum viscosity at low temperatures is reduced since too viscous an automatic transmission fluid will not adequately flow as needed to actuate the hydraulic valves and other hydraulic mechanisms of the automatic transmission. Automobile manufacturers have recently changed their specifications for automatic transmission fluid to require lower maximum viscosities at low temperatures. The next generation automatic transmissions will require next generation automatic transmission fluids, especially to be sure the automatic transmission fluid flows adequately at low temperatures. For example, new generation automatic transmission fluids must have Brookfield viscosities at -40.degree. C. of less than 10,000 cP, 13,000 cP, or 17,500 cP. Current requirements for automatic transmission fluids typically only require a Brookfield viscosity at -40.degree. C. of less than 20,000 cP.
Synthetic lubricants made from polyalphaolefins ("PAO's") and some new unconventional high viscosity index mineral base oils can be used to meet these new viscometric requirements. However, those are expensive to manufacture. It would be advantageous to have a relatively inexpensive mineral oil-based lubricant that can lower the cost of meeting the new viscometric requirements. The lubricating composition of the present invention meets this need.